All Skype-to-Skype voice, video, file transfers and instant messages are encrypted. This protects you from potential eavesdropping by malicious users.
If you make a call from Skype to mobile and landline phones, the part of your call that takes place over the PSTN (the ordinary phone network) is not encrypted.
For example, in the case of group calls involving two users on Skype-to-Skype and one user on PSTN, then the PSTN part is not encrypted, but the Skype-to-Skype portion is.
For instant messages, we use TLS (transport-level security) to encrypt your messages between your Skype client and the chat service in our cloud, or AES (Advanced Encryption Standard) when sent directly between two Skype clients. Most messages are sent both ways, but in the future it will only be sent via our cloud to provide the optimal user experience.
Voice messages are encrypted when they're delivered to you. However, after you have listened to a voice message, it is transferred from our servers to your local machine, where it is stored as an unencrypted file.
Skype uses the AES (Advanced Encryption Standard*), also known as Rijndael, which is used by the US Government to protect sensitive information, and Skype has for some time always used the strong 256-bit encryption. User public keys are certified by the Skype server at login using 1536 or 2048-bit RSA certificates.
*Skype is not responsible for the content of external sites.
To learn more about encryption, please visit our Security Center.